Many integral membrane proteins are hetero-oligomers in situ. Their biogenesis requires the targeting of different subunit polypeptides to the correct intracellular membrane and their proper assembly into a functional complex. Previous studies have stressed the importance of NH2-terminal "leader peptides" for both the targeting of proteins to membrane surfaces and their subsequent insertion into the membrane. They have also made clear that proteins which are inserted into or through membranes co-translationally use their "leader-peptides", in conjunction with soluble proteins, to regulate their own translation. Despite the obvious importance of "leader peptide" presequences, their fate, after being processed is uncertain. Also uncertain is whether they perform other functions in membrane biogenesis. We propose to test a new hypothesis: that "leader peptides" function as subunits in some membrane protein oligomers. This hypothesis has grown out of the finding that three recently sequenced nuclear-coded polypeptide subunits (VII, VIIa, and VIII) of yeast cytochrome c oxidase are homologous to known "leader peptides" from other, unrelated, nuclear-coded mitochondrial proteins. We plan to 1) clone and sequence the structural genes for these three subunits to determine if they are, in fact, "leader peptides" derived from larger polypeptide precursors extending from their -COOH terminus; 2) determine, by constructing null mutants in their structural genes, if they are required for the assembly or function of holocytochrome c oxidase; and 3) identify proteins with which they are contiguous should we find that they are "leader pepdides". The studies may identify another role for "leader peptides" in membrane biogenesis and provide an important clue regarding the mechanisms by which the assembly pathways of protein oligomers in the same membrane are coordinated.